1. Field of the Invention
This invention relates to a semiconductor switch circuit used in a telephone exchange system or the like and employing 4-terminal PNPN switches capable of passing a continuous AC signal.
2. Description of the Prior Art
In a conventional semiconductor switch circuit employing 4-terminal PNPN switches for switching an AC signal, a couple of such PNPN switches are connected in inverse-parallel relation to constitute a bilateral switch, and current is supplied from an associated drive circuit into the cathode gates of these two PNPN switches to permit passage of the AC signal. However, the conventional semiconductor switch circuit with such a construction has been disadvantageous in that the drive voltage of the drive circuit is required to exceed the maximum value of the signal voltage of the transmitted AC signal depending on the load condition.
In an effort to obviate such a disadvantage, an improved semiconductor switch circuit employing a 4-terminal PNPN switch for switching an AC signal has previously been proposed by the inventors in Japanese Laid-Open Publication No. 10012/77 published Jan. 26, 1977. This improved semiconductor switch circuit has a construction as shown in FIG. 1.
The proposed semiconductor switch circuit shown in FIG. 1 comprises a current-supplying constant-current circuit 2 including transistors Q.sub.1 and Q.sub.2, a resistor R.sub.1 and level shift diodes D.sub.4 ; a current-sinking constant-current circuit 3 including a transistor Q.sub.3, a resistor R.sub.2 and level shift diodes D.sub.5 ; a 4-terminal PNPN switch 1; a current change-over circuit 4 including reverse current blocking diodes D.sub.1, D.sub.1 ' and D.sub.2 ; and reverse-current blocking diodes D.sub.3 and D.sub.3 '. In this prior art semiconductor switch circuit, the current change-over circuit 4 distributes the output current of the current-supplying constant-current circuit 2 to the cathode gate G.sub.K of PNPN switch 1 and to the current-sinking constant-current circuit 3 depending on the potential at the cathode of PNPN switch 1. When the potential at the cathod K of PNPN switch 1 is negative, current flows into the cathode gate G.sub.K to turn on the PNPN switch 1, while when the potentials at the cathode K and anode A are both positive, current flows out from the anode gate G.sub.A to turn on the PNPN switch 1. Although not shown in FIG. 1, the diodes D.sub.1 ' and D.sub.3 ' are connected to the cathode gate and anode gate respectively of another PNPN switch connected in inverse-parallel relation to the illustrated PNPN switch 1. A control circuit including a power supply E.sub.1, a switch SW.sub.1 and a resistor R.sub.3 is illustrated in FIG. 1 to facilitate understanding of the circuit operation. It is apparent however that this control circuit is actually an electronic circuit.
While the proposed semiconductor switch circuit shown in FIG. 1 has been satisfactory in obviating the aforementioned prior art defect, it has still been defective in that many elements thereof have to be separately electrically isolated from one another during integration into an integrated circuit form, and thus the desired high integration density cannot be achieved. For example, in the case of the diodes D.sub.1, D.sub.1 ' and D.sub.2 which are common-connected at one end thereof, their anodes can be formed in a common P-type semiconductor region, but their cathodes must be formed in independent N-type semiconductor regions. Further, in this case, a high breakdown voltage is required for these diodes D.sub.1, D.sub.1 ' and D.sub.2 since the voltage of the load circuit is sometimes applied to these diodes D.sub.1, D.sub.1 ' and D.sub.2.